Cumulus clouds with flat bottoms...ever wonder why?

Up until today, I never wonder about that, but I don’t why today, while in the University I looked out off the window to the city and a saw a very distinguished line separating the clouds in the sky... So I wonder why that happens! First step discover what kind of cloud I saw in the sky. That wasn’t so hard. The several types are divided for both shape related and altitude related characteristics. The ones I saw are the low altitude ones the cumulus, so called like this because they appear to be in a pile or heap.So my question got a little more specific...Why do Cumulus clouds have flat bottoms?After a little more research, and thanks to Yahoo Answers I found my answer:

It has to do basically with the temperature of the air and the point when the water condensate in to small droplets to form the clouds. Since the temperature in the atmosphere does not vary horizontally, just vertically the clouds appear to have flat bottom. The bottom of the cloud represents the so called dew point, above which the water condensates and below which the water remains in vapour!